Re: [PATCH v4 2/2] virtio-blk: support mq_ops->queue_rqs()

[Christoph Hellwig <hch@infradead.org>]

On Tue, Apr 05, 2022 at 02:31:22PM +0900, Suwan Kim wrote:
> This patch supports mq_ops->queue_rqs() hook. It has an advantage of
> batch submission to virtio-blk driver. It also helps polling I/O because
> polling uses batched completion of block layer. Batch submission in
> queue_rqs() can boost polling performance.
> 
> In queue_rqs(), it iterates plug->mq_list, collects requests that
> belong to same HW queue until it encounters a request from other
> HW queue or sees the end of the list.
> Then, virtio-blk adds requests into virtqueue and kicks virtqueue
> to submit requests.
> 
> If there is an error, it inserts error request to requeue_list and
> passes it to ordinary block layer path.
> 
> For verification, I did fio test.
> (io_uring, randread, direct=1, bs=4K, iodepth=64 numjobs=N)
> I set 4 vcpu and 2 virtio-blk queues for VM and run fio test 5 times.
> It shows about 2% improvement.
> 
>                                  |   numjobs=2   |   numjobs=4
>       -----------------------------------------------------------
>         fio without queue_rqs()  |   291K IOPS   |   238K IOPS
>       -----------------------------------------------------------
>         fio with queue_rqs()     |   295K IOPS   |   243K IOPS
> 
> For polling I/O performance, I also did fio test as below.
> (io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=4)
> I set 4 vcpu and 2 poll queues for VM.
> It shows about 2% improvement in polling I/O.
> 
>                                       |   IOPS   |  avg latency
>       -----------------------------------------------------------
>         fio poll without queue_rqs()  |   424K   |   613.05 usec
>       -----------------------------------------------------------
>         fio poll with queue_rqs()     |   435K   |   601.01 usec
> 
> Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> ---
>  drivers/block/virtio_blk.c | 110 +++++++++++++++++++++++++++++++++----
>  1 file changed, 99 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 712579dcd3cc..a091034bc551 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -92,6 +92,7 @@ struct virtio_blk {
>  struct virtblk_req {
>  	struct virtio_blk_outhdr out_hdr;
>  	u8 status;
> +	int sg_num;
>  	struct sg_table sg_table;
>  	struct scatterlist sg[];
>  };
> @@ -311,18 +312,13 @@ static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
>  		virtqueue_notify(vq->vq);
>  }
>  
> -static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> -			   const struct blk_mq_queue_data *bd)
> +static blk_status_t virtblk_prep_rq(struct blk_mq_hw_ctx *hctx,
> +					struct virtio_blk *vblk,
> +					struct request *req,
> +					struct virtblk_req *vbr)
>  {
> -	struct virtio_blk *vblk = hctx->queue->queuedata;
> -	struct request *req = bd->rq;
> -	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> -	unsigned long flags;
> -	int num;
> -	int qid = hctx->queue_num;
> -	bool notify = false;
>  	blk_status_t status;
> -	int err;
> +	int num;
>  
>  	status = virtblk_setup_cmd(vblk->vdev, req, vbr);
>  	if (unlikely(status))
> @@ -335,9 +331,30 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  		virtblk_cleanup_cmd(req);
>  		return BLK_STS_RESOURCE;
>  	}
> +	vbr->sg_num = num;

This can go into the nents field of vbr->sg_table.

> +	int err;
> +
> +	status = virtblk_prep_rq(hctx, vblk, req, vbr);
> +	if (unlikely(status))
> +		return status;
>  
>  	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> -	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
> +	err = virtblk_add_req(vblk->vqs[qid].vq, vbr,
> +				vbr->sg_table.sgl, vbr->sg_num);

And while we're at it - virtblk_add_req can lose the data_sg and
have_data arguments as they can be derived from vbr.